Alkali Metal Cations Influence the CO 2 Adsorption Capacity of Nanosized Chabazite: Modeling vs Experiment

The CO 2 adsorption behavior of nanosized chabazite (CHA) containing different compositions of extra-framework alkali metal cations (Na + , K + , Cs +) with a constant Si/Al ratio of 2.2 was investigated experimentally (adsorption isotherms and in situ FTIR spectroscopy) and theoretically by Grand Canonical Monte Carlo (GCMC) modeling. The CO 2 adsorption isotherms were in a good agreement with the GCMC modeling in the pressure range of 0-60 KPa for all samples except the Cs + containing nanosized CHA. The GCMC modeling did not consider the kinetics of adsorption thus the CO 2 capacity for the Cs-CHA sample was overestimated; the partial blockage of the pores and inaccessibility of CO 2 to the free voids of the CHA structure was experimentally observed by recording adsorption isotherms and in situ FTIR study. The FTIR results revealed the CO 2 adsorbed mainly physically on all samples with the sorption capacity decreasing in the following order: Na-CHA ≃ K-CHA > Cs-CHA. The Na-CHA demonstrated significant micropore volume compared to the other samples as measured by N 2 physisorption due to the inability of Na + to effectively occupy the 8MRs. This study implies that the different alkali metal cation forms of the nanosized CHA zeolite are of practical interest for flue gas purification where a high CO 2 /N 2 selectivity is required